1. Field of the Invention
This invention relates to a plastic encapsulated semiconductor device and a structure for mounting the same devices, and more particularly to a plastic encapsulated semiconductor device suitable for the reduction of the thermal resistances of devices of this kind, and a structure mounted with such devices.
2. Description of the Prior Art
The most popular means for reducing the thermal resistance of a plastic encapsulated semiconductor device is a method of fixing metallic radiation fins to a package with a bonding agent as disclosed in Japanese Patent Laid-Open Nos. 105354/1980 and 63952/1985. Although according to this method there can be the performance of fins, it has problems concerning the reliability of the bonded surfaces of the fins and the productivity of fin-bonded packages.
As to methods of providing a package with plastic fins formed integrally with a package body as means for solving these problems, the following are exemplary: Japanese Patent Laid-Open Nos. 129350/1981 and 81735/1987 and Japanese Utility Model Laid-Open Nos. 104553/1986 and 84958/1988.
A method of providing a package surface with a plurality of projections and recesses for the purpose of increasing the heat-transferring area of the package surface and thereby reducing the thermal resistance thereof is disclosed in Japanese Utility Model Laid-Open No. 31541/1988.
Out of these conventional techniques, the method of bonding fins to a package has a problem concerning the reliability of the fin-bonded surfaces as already mentioned. In general, the coefficient of linear expansion of a fin material (mainly a metal) and that of a plastic with which a package is encapsulated are different. This causes thermal stress to occur in the layer of bonding agent, and this layer is therefore liable to break.
In the production of surface mounting type packages the demand for which has increased in recent years, a package body is heated to not lower than 200.degree. C. when the leads and the substrate are soldered to each other. At such a high temperature, the strength of a bonding agent decreases considerably, so that it is even more difficult to secure the reliability of the product. In a package of such construction, the number of parts and manufacturing steps required increase, and the productivity thereof decreases.
The techniques for providing a package with plastic fins formed integrally with a package body, for eliminating the drawbacks of the above-mentioned techniques, are known as described before. A structure having such plastic fins is inferior in the cooling performance relative to a structure using metallic fins. However, the former structure does not lower the above-mentioned reliability of the product since it does not have a layer of bonding agent. It also has a high productivity since the number of manufacturing steps does not increase. However, packages of this construction have not yet been used practically. The reasons reside in the following two fatal drawbacks of the packages.
A perspective of a conventional plate type plastic fin-carrying package is shown in FIG. 11. Leads 3 are extended outwardly from side surfaces of a package body 2, and plastic fins 1a are provided on the upper surface thereof. It is very difficult to form a package of such a shape by a generally-used transfer molding.
The flow of a plastic in a transfer molding of a conventional construction will be described with reference to FIG. 12. FIG. 12 shows in longitudinal section including fins the flow of a plastic during the transfer molding of a package. A lead frame 5 mounted with a chip 4 is inserted between metal molds 8, 9, and a molten plastic 6 is injected through a gate 10 on the left-hand side. Since a flow passage on the lower side of the lead frame 5 is much narrower than that on the upper side thereof, it has a high resistance, and the molten plastic does not smoothly flow therein. As a result, the plastic 6 flows only in the passage on the upper side of the lead frame, and an unfilled space 7a remains on the lower side of the lead frame.
FIG. 13 is a cross sectional view where the section is perpendicular to that of FIG. 12. Since the plastic 6 flows only into the wider passages of the fin side, the unfilled space 7a remains in the portions having a high flow passage resistance.
Even if the filling of plastic by a transfer molding is done perfectly during the production of such a conventional structure, various defects are liable to occur in the package. FIG. 14 is a longitudinal cross sectional view of a package including fins which is formed after the completion of the molding step. The coefficient of linear expansion of the plastic 6 is higher than that of the chip 4, and shrinkage occurs in the plastic 6 during the curing process. In the structure shown in FIG. 14, in which the upper and lower portions are unbalanced, the fins shrink greatly to cause curvature as shown in the drawing. Since bending stress occurs in the lower surfaces of the chip 4 and package during this time, cracks 11a, 11b readily occur therein. A delaminated portion 12 is also apt to occur in the interface between the plastic 6 and lead frame 5.
In a package provided with cylindrical fins as disclosed in Japanese Utility Model Laid-Open Nos. 104553/1986 and 84958/1988, a plastic does not smoothly flow into the cylindrical portions, and unfilled spaces remain in these portions, so that it is difficult to form cylindrical fins.
In order to eliminate the drawbacks of the packages with fins according to the conventional techniques described above, the height of the fins must be reduced. However, the reduction of the height of fins causes a great decrease of the radiation effect of the fins, and makes it difficult to attain the prime object of reducing the thermal resistance of the package.
When only projections and recesses are provided in the surface of a package as shown in Japanese Utility Model Laid-Open No. 31541/1988, the air does not flow into the recesses. Therefore, these portions become nearly adiabatic, and the thermal resistance-reducing effect of the package is small.
An described above, in a conventional package having plastic fins formed integrally therewith, consideration is not satisfactorily given to the productivity and reliability of the product.